Note: MetaMask recently updated the ChainID to be a required field. When you update, you may need to reenter the chain id 100, and it will be converted to a hexidecimal: 0x64.
If you are having issues, try entering 100 for Chain ID and resaving the configuration.
If you'd prefer not to make these changes, Nifty Walletprovides a built-in xDai RPC.